Stentor coeruleus is an animal belonging to the family of protozoa which exhibits photophobic behavior (i.e., swimming away from light). The action spectrum for photophobic response suggests that the photoreceptor involved is stentorin, which has the hypericin chromophore, an antidepressant drug. It is, thus, interesting that this aneural protozoa utilizes the neuropsychotherapeutic agent for its own sensory transduction and defense against photodynamic effects of light. We propose to study excited state properties of both the photoreceptor model (hypericin) and stentorin of Stentor coeruleus in order to elucidate the possible molecular mode of its action in the photosensory transduction process. We propose the proton gradient formation across the stentorin photoreceptor-bound granular membrane as a primary molecular mechanism of the light-induced motility of stentor. Stentorin-liposomes and stentorin photoreceptor proteins (including membranes) will be prepared and charactorized. The mechanism of proton release from the light-excited states of these preparations will be investigated by means of nano- and pico-second time-resolved fluorescence and other spectroscopic methods. Light-triggered pH gradient formation across whole stentor cell membranes will also be studied.